This invention relates generally to waveguide filtering devices, and more particularly, to filters for use with systems operating in the millimeter wavelength region of the electromagnetic spectrum. Waveguide filters, particularly bandpass filters, have a wide range of applications, for example in communication systems and radar systems. Conventional waveguide filters for these purposes are significantly limited by their small physical size, which increases the difficulty of manufacture, and relatively high losses. An alternative to the use of waveguide filters is a quasi-optical bandpass filter, apparently first proposed in 1974 by Adel Saleh, in a paper entitled "An Adjustable Quasi-Optical Bandpass Filter," published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-22, No. 7, July 1974, pp. 728-39.
In this paper, the author pointed out that filtering operations at millimeter and submillimeter wavelengths, and in the far infrared region of the spectrum, could be performed at low loss and high power handling capability by employing a quasi-optical technique in free space rather than a conventional filter in a waveguide. The simplest and most common form of quasi-optical filter is a Fabry-Perot resonator employing two or more metallic grids for filtering. Such a filter has no provision for adjusting the bandwidth or shape of the frequency response. Accordingly, precise construction is required to provide the desired characteristics, and changes are difficult to effect.
Saleh proposed a plane-wave quasi-optical filter comprising three or more wire-grid polarizers whose planes are parallel and are spaced at intervals of typically a quarter wavelength. The bandwidth and shape of the frequency response of the filter can be adjusted by changing the relative angular orientations of the polarizers, without affecting the center frequency of the device. Saleh assumed in the first part of his paper that each wire-grid element was an ideal polarizer, i.e., an incident wave would be totally reflected if its electric field vector were parallel to the wires of the grid, and would be totally transmitted if the electric field vector were perpendicular to the wires of the grid. The mathematics of this type of filter are described in detail in the Saleh paper. Basically, the overall response characteristic of the device is developed by considering the operation each basic filter section, comprising two adjacent wire-grid polarizers. Each basic section has a reflection characteristic and a transmission characteristic. Although one such section cannot have one-hundred-percent transmission at any frequency, assuming the grids are oriented at different angles, a series of basic sections can provide for total transmission at certain frequencies, and total reflection at others, at least in theory.
The theory of the quasi-optical filter was further developed by the present inventor in two subsequent papers. In "The Network Representation and the Unloaded Q for a Quasi-Optical Bandpass Filter," IEEE Trans. Microwave Theory and Techniques, Vol. MTT-27, April 1979, pp. 355-60, the author presented a technique for computing the loss characteristic of quasi-optical filters. In "Design Formulas for a Quasi-Optical Diplexer or Multiplexer," IEEE Trans. Microwave Theory and Techniques, Vol. MTT-28, April 1980, pp. 363-68, formulas are developed for performing diplexing and multiplexing operations using quasi-optical filter devices.
Quasi-optical filters based on those first described by Saleh have a significant disadvantage when applied to systems operating in the millimeter wavelength range. The quasi-optical devices operate on the principle that the incident waves are plane waves travelling in free space. In typical applications, the incident waves will not be plane waves, and the aperture of each polarizer element may have to be as large as one hundred wavelengths across to simulate plane-wave operation. At millimeter wavelengths, this is a most inconvenient limitation. Another drawback to the use of quasi-optical filters is that plane-wave launching devices, such as parabolic reflectors, are required, one on each side of the filter device.
It will be appreciated from the foregoing that there is still a significant need for a filter device, particularly one that is operable in the millimeter wavelength range, providing the convenience of adjustability of the quasi-optical filter, but without the inherent disadvantages of that device. The present invention fulfills this need.